Wellheads are used in oil and gas drilling to suspend casing strings, seal the annulus between casing strings, and provide an interface with the blowout preventer (“BOP”). The design of a wellhead is generally dependent upon the location of the wellhead and the characteristics of the well being drilled or produced.
In drilling the well, it is conventional to pass a number of concentric tubes, or casings, down the well to support the borehole and/or isolate the borehole from fluid producing zones. An outermost casing is fixed in the ground, and the inner casings are each supported from the next outer casing by casing hangers which take the form of inter-engaging internal shoulders on the outer casing and external shoulders on the inner casing. The wellhead is thus used to support a number of casing hangers that support the weight of the casing.
Typically, such casing hangers are fixed in position on each casing and positioned in the wellhead. However, a fixed position casing hanger might be unsatisfactory if the hang-off point of one casing on another may need to be adjusted. Additionally, even if using fixed position bowl-type casing hangers, a casing may become stuck as it is being run in the well and thus the fixed casing hanger is not in position to support the casing string. In such cases, slip-type supports may be used to support the casing instead of the fixed position casing hanger.
Slip supports are friction wedges that “grip” the casing string and use “teeth” to bite into the casing when subjected to actuating force. Seal assemblies may then be used to seal the annulus between the casing and the wellhead. However, the seals as well as the casing itself are subject to forces throughout the life of the well that might cause the slip hanger to unseat. Any resulting travel of the casing or the seal assembly may compromise the seal between the casing and the wellhead. Thus, the slips and the seals used with slip-type casing hangers must be restrained from movement when subjected to force. As such the seal assemblies typically include robust bodies including both inner and out seals that are set upon actuation torque from a tool above the seal assembly. However, because the torque is applied from above the seal assembly, the actuator tool may only access one portion of the seal assembly for the actuation torque. Thus, usually both the inner and outer seals of the seal assembly are set simultaneously. In some situations, however, the inner and outer seals require different amounts of force to be set and thus simultaneous actuation constrains the ability to properly form a seal against the wellhead.